Presently many large automotive vehicles such as trucks and buses utilize drum type braking systems for stopping the vehicles. The systems may be actuated hydraulically or may be actuated by a combination of pneumatics and hydraulics. These braking systems include a plurality of brake shoes each having at least one brake block of friction material mounted on a backing plate by means of rivets or by bonding. Typically a pair of brake shoes mounts on a fixture inside of the rotating hub of a vehicle wheel. During the braking operation the two brake shoes are moved radially outwardly of the fixture and into engagement with the inside surface of a brake drum which mounts on the hub and overlies the brake shoe. Friction resulting from the engagement of the outer surface of the brake blocks with the inner surface of the brake drum causes the rotation of the vehicle wheels to slow and ultimately to stop.
The frictional engagement between the brake blocks and the brake drums causes the outer surface of the brake blocks to erode or wear. Eventually the brake blocks become worn to where they are unusable. This occurs when a brake block has been worn to where only a minimum amount of friction material overlying the tops of the rivets, or adhesive composition which affix the block to the brake drum has been reached. If the brake block is permitted to remain on the vehicle after being worn to where only the minimum safe amount of friction material remains, any additional wear of the brake block will cause the rivets or bolts to become exposed and begin to scrape against the inner surface of the brake drum or cause the composition interface to scrape against the inner surface of the brake drum with a complete absence of friction material. Either of these conditions is undesirable and may result in diminished braking capability and in damage to a brake drum.
In order to check the amount of wear of a friction material brake block on a brake shoe to determine if it has been eroded to the minimum safe amount of remaining material, in the past it has been necessary to remove the wheel and brake drum from the axle of the vehicle to visually inspect the brake block. After the brake drum has been removed from the vehicle the thickness of the brake block material may be measured to determine the amount of material remaining on the brake shoe. In some instances an arcuate line may be scribed on the sides of the brake block to indicate the safe minimum thickness of the block which makes it unnecessary to measure the thickness of the block. In some instances the personnel servicing the vehicle do not measure the thickness of the brake block but rely on experience to determine if sufficient material remains to enable a brake block to be used safely. Even where the service personnel measure the thickness of the brake block manually they first must determine if the brake block is bonded or riveted onto the backing plate in order to ascertain properly the exact thickness of the usable remaining friction material on the brake block. Because of the expense, inconvenience and vehicle downtime which results when the wheels must be removed from a vehicle for inspection of the brake blocks, vehicle operators may make such inspections less often than would be required to find a brake block which has been worn out. If this occurs, the braking capacity of the vehicle may be diminished and a brake drum may be damaged.
It is desirable to provide a friction material brake block having a wear indicator means by which the thickness of the remaining material on a brake block which may be utilized safely may be observed without having to remove the wheel and the brake drum from the vehicle.